1. Field of the Invention
The present invention relates to an illuminating optical system adapted for use in semiconductor device manufacture, and an exposure apparatus utilizing such optical system.
2. Related Background Art
Reduction projection exposure apparatus have been employed for the manufacture of semiconductor devices with ultrafine patterns such as LSI (large scale integrated circuits) or VLSI (very large scale integrated circuits), and great efforts have been continued to achieve transfer of finer patterns. For responding to such reduction in the dimension of patterns, there has been explored the increase in the numerical aperture (N.A.) of the projection optical system, together with the use of a shorter wavelength in the exposing light, and projection optical systems with N.A. exceeding 0.5 have been realized.
In the actual projection exposure utilizing the projection optical system with such large numerical aperture, the optimization of illuminating conditions is an important factor. For this reason there is proposed, for example in the Japanese Patent Laid-open Application No. 59-155843, to regulate socalled .sigma.-value corresponding to the ratio of the N.A. of the illuminating optical system to that of the projection optical system, in order to obtain an appropriate balance between the resolving power and the contrast.
In said proposal, a variable diaphragm with a variable aperture is positioned at the exit side of a fly's eye lens where a secondary light source image is formed in the illuminating optical system, and the size of the light source image is controlled by varying the size of aperture of said variable diaphragm. Said .sigma.-value corresponds to the ratio of the size of the light source image of the illuminating optical system, formed on the pupil of the projection optical system, to the size of said pupil of the projection optical system. Therefore, the numerical aperture of the illuminating optical system has been practically varied by a change in the size of the light source image, through a change in the level of light shielding on an actually formed light source image, and the optimization of the illuminating condition, or of the .sigma.-value, has been achieved so as to optimize the balance of the resolving power and the contrast, according to the size of the pattern to be projection exposed.
In such optical apparatus, there is generally required a high illumination intensity on the illuminated plane (mask or reticle), in order to improve the throughput at the exposure of fine patterns.
However, in the above-explained conventional apparatus, when the aperture of the variable diaphragm is reduced in order to optimize the illuminating condition, or the .sigma.-value, for obtaining an optimum balance of the resolving power and the contrast on a predetermined pattern according to the fine pattern to be projection exposed, there will result a significant loss in the amount of light in the peripheral part of the secondary light source image, due to the shielding by the variable diaphragm. Thus, although the illumination intensity on the illuminated plane (mask or reticle) becomes maximum when the .sigma.-value is selected largest (when the aperture of the variable diaphragm is made largest), said illumination intensity becomes lower when a smaller .sigma.-value is selected (when the aperture of the variable diaphragm is made smaller), thus leading to a fatal drawback of a reduced through-put.